Improvement of safety parameters by using erbium-loaded canflex fuel bundle in the candu reactor

Abstract

A relatively weak Doppler effects and positive Coolant Temperature Coefficient (CTC) lead to a near-zero, or slightly positive power coefficient of reactivity (PCR) in CANDU. To ensure the stability of CANDU reactors in a power transient situation, the PCR should be less than zero at operating conditions. In this study, the CANFLEX fuel bundle is introduced to improve these safety parameters. Due to the reduced linear power in CANFLEX, the fuel temperature is lowered and the fuel temperature coefficient (FTC) is enhanced. In addition, a slightly increased coolant flow area of CANFLEX can be advantageous in that the coolant boiling can be effectively retarded in the CANDU core. In this study, important safety parameters such as FTC, CTC, and PCR are evaluated for the two standard and CANFLEX fuels. The analysis of the safety parameters are done with the continuous energy Monte Carlo code Serpent2 based on ENDF/B-VII data library. For a more accurate FTC evaluation, the Doppler Broadening Rejection Correction (DBRC) option is used to describe the thermal motion of U-238. In order to further improve the safety parameters and PCR value, an Er burnable poison is loaded into the central fuel pin of the CANFLEX fuel bundle. We have investigated the feasibility of loading the Er absorber without compromising the fuel discharge burnup by removing parts of the adjusterrods, which are fully inserted during the normal operation of CANDUreactors. We have shown that the safety parameters can be noticeably improved by using the limited loading of the Er burnable poison.